In a Voice over Internet Protocol (VoIP) network, establishment of an end-to-end call may involve multiple communication legs each providing its own signaling mechanisms. For example, when a Unified Messaging System (UMS) operating in an IP Multimedia Subsystem (IMS) network receives a request from a subscriber to browse voicemails, a common option of the UMS is to provide the subscriber an auto-dial feature to respond to the voicemail message without terminating the call. When the call is outside of the Local Access and Transport Area (LATA) of the UMS, the UMS (due to regulatory reasons) has to re-direct the call to an intermediate system such as a Calling Card Server (CCS) to place the call.
When the CCS places the call to the Called Endpoint, it will typically receive call progress tones via signaling messages. The CCS must then convert those call progress tones into in-band tones for the Calling Endpoint. The UMS under these circumstances does not know whether the Called Endpoint is ringing or a busy signal is being transmitted. This situation can limit the UMS from providing the calling party additional supplemental communication features during the call set-up process.